1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) display device manufacturing method and to an organic EL display device.
2. Description of the Related Art
Image display devices (organic EL displays) using organic electroluminescence elements (OLED: Organic Light-Emitting Diodes) are well-known as image display devices using current-driven light-emitting elements. Due to such advantages as excellent viewing angle characteristics and low power consumption, such organic EL displays have gained much attention as candidates for next-generation flat panel displays (FPDs).
In organic EL display devices, organic EL elements included in pixels are normally arranged in a matrix. In an organic EL display referred to as a passive-matrix organic EL display, an organic EL element is provided at each crosspoint between row electrodes (scanning lines) and column electrodes (data lines), and such organic EL elements are driven by applying a voltage equivalent to a data signal, between a selected row electrode and the column electrodes.
On the other hand, in an organic EL display device referred to as an active-matrix organic EL display, a thin film transistor (TFT) is provided in each crosspoint between scanning lines and data lines, the gate of a drive transistor is connected to the TFT, the TFT is turned ON through a selected scanning line so as to input a data signal from a data line to the drive transistor, and an organic EL element is driven by such drive transistor.
Unlike in the passive-matrix organic EL display where, only during the period in which each of the row electrodes (scanning lines) is selected, does the organic EL element connected to the selected row electrode emit light, in the active-matrix organic EL display, it is possible to cause the organic EL element to emit light until a subsequent scan (selection), and thus a reduction in display luminance is not incurred even when the number of scanning lines increases. Therefore, since driving with low voltage is possible, reduction of power consumption becomes possible. However, in the active-matrix organic EL display, due to variation in the characteristics of drive transistors and organic EL elements arising in the manufacturing process, the luminance of the organic EL elements are different among the respective pixels even when the same data signal is supplied, and thus there are instances where luminance unevenness, such as a band or unevenness, occurs.
In response, there is proposed a correction method of correcting bands and unevenness occurring in an organic EL display in which, by correcting an image signal (data signal), the luminance of the organic EL elements corresponding to the image signal supplied to the respective pixels can be corrected to a predetermined standard luminance (for example, Patent Reference 1: Unexamined Japanese Patent Application Publication No. 2005-284172).
In the correction method of Patent Reference 1, by measuring the luminance distribution or current distribution of at least three gradation levels in each pixel of an organic EL display, it is possible to obtain the gain and offset which are correction parameters for correcting the luminance of the organic EL element corresponding to the image signal supplied to the respective pixels to a predetermined standard luminance.
However, the conventional correction methods have the problems described below.
Conventionally, as a correction method, there is for example a method of obtaining gain and offset, which are correction parameters, using the least-square technique. In this method which uses the least-square technique, multi-gradation level luminance measurement is performed for each pixel, and the gain and offset are obtained using a predetermined calculation method, based on the luminance difference between the luminance of each pixel obtained in each measurement and the representative voltage-luminance characteristics. As an example, luminance L1 to L6 at the six points of voltages V1 to V6 is measured for a certain pixel using the least-square technique, and V×1 to V×6 are obtained as the correction parameters, as shown in FIG. 1.
However, in the correction method which uses the least-square method for example, by nature it is necessary to perform the luminance measurement on each pixel for a number of gradation levels that is at least 3 gradation levels and preferably 5 gradation levels or more, and thus there is the problem of requiring time from the performance of the luminance measurement for each pixel up to the obtainment of the correction parameters. In particular, a very long time is required for the luminance measurement in the low gradation-side. As a result, there is the problem that measurement tact from the performance of the luminance measurement for each pixel up to the obtainment of the correction parameters becomes long.
Furthermore, in an organic EL display, there is a tendency for the occurrence of streaky luminance unevenness and so on in the low gradation regions. The human eye recognizes luminance differences more easily in the low gradation-side than in the high gradation-side. As such, it is preferable that correction precision be higher for the low gradation-side than the high gradation-side. However, normally, the luminance difference between the representative voltage-luminance characteristics and the voltage-luminance characteristics of each pixel increases as one goes further into the high gradation-side, and since the least-square method simultaneously obtains the gain and offset by calculation so that the luminance error in the high gradation-side is minimized, there is the problem that, although the correction error in the high gradation-side can be minimized, the correction error in the low gradation-side becomes big compared to that in the high gradation-side.